A typical system for transmitting cellular signals within a cellular network, such as system 10 shown in FIG. 1, includes a base station 12, a tower 14, and antennas 16 mounted on the tower 14, often near the top of the tower 14. The base station 12 has a set of radios 18 for receiving telephony signals from the public switched telephone network (PSTN) through a mobile switching center (MSC) and for generating corresponding radio signals. A set of high power amplifiers 20 amplify the radio signals and supply the amplified radio signals over feeder lines 22 to the antennas 16. The antennas 16 generate electric fields which are propagated throughout the radiation patterns of the antennas 16 so that cellular mobile radiotelephones within a cell associated with the antennas 16 may receive the signals. The antennas 16 are preferably mounted well above the ground so as to increase the range of the antennas 16, thereby decreasing the number of required cells within the cellular network. Although the tower 14 has been shown with two antennas 16, the tower 14 may have other numbers of antennas 16 and corresponding feeder lines 22.
The system 10 for transmitting cellular signals suffers from a disadvantage that losses between the antennas 16 and the base station 12 are relatively large. The tower 14 is typically a fairly tall structure so as to increase the range of the antennas 16. As a result, the feeder lines 22 typically must span great distances before reaching the antennas 16. Due to the losses associated with the feeder lines 22 and the great distances over which the feeder lines 22 typically must span, the feeder lines 22 introduce a significant loss in signal strength to the signals supplied to the antennas 16. To reduce these losses, the feeder lines 22 are preferably low-loss cables, such as pressurized coaxial cable. Even with low-loss feeder lines 22, however, the feeder lines 22 nonetheless still introduce a large amount of loss because of the great distances involved between the base station 12 and the tower 14 and between the base of the tower 14 and the antennas 16. For instance, a 400 feet 15/8 inch coaxial line typically introduces a 4 dB insertion loss for a 2 GHz signal.
In view of these losses in the feeder lines 22, the base station 12 must supply high power signal signals so that the antennas 16 receive signals which are at a sufficiently high power level to reach all cellular mobile radiotelephones within the cell. The base station 12 therefore has amplifiers 20 which operate at a high power level to boost the power level of the radio signals to a level which is sufficient, after considering the losses over lines 22, for the antennas 16 to broadcast signals to the cellular mobile radiotelephones within its cell.
The losses on the feeder lines 22 are not constant but rather fluctuate due to a number of factors, including the temperature of the feeder lines 22. As the temperature decreases, the resistivity of the feeder lines 22 increases, thereby increasing the losses over the feeder lines 22. Since the magnitude of the losses vary, the magnitude of the signals reaching the antennas 16 and the magnitude of the signals transmitted by the antenna 16 would vary accordingly. The antennas 16 preferably operate at the highest permissible power level to maximize the strength of the signals received by the cellular mobile radiotelephones but are often forced below the preferred level due to fluctuations in losses. For example, the amplifiers 20 often comprise 100 to 200 watt multi-channel amplifiers or 50 watt single channel amplifiers. The conventional system 10 therefore suffers from a disadvantage in that it transmits signals at less than optimal levels.
The system 10 is relatively expensive considering the measures necessary to supply the antennas 16 with signals at a sufficiently high power level. These measures, as described above, include low-loss lines 22, which add a substantial cost due to the large distances over which the feeder lines 22 travel, and the amplifiers 20, which are costly due to the high power levels at which they operate. These costs for the low-loss feeder lines 22 and the high power amplifiers 20 are multiplied for an entire cellular network by the number of antennas 16 per tower 14 and also by the number of towers 14 within the cellular network.
In addition to a high cost, a further disadvantage of the system 10 is that it has a relatively high failure rate. As set forth above, the signals supplied over the lines 22 to the antennas 16 must be at a sufficiently high power level to overcome losses associated with the feeder lines 22. These high power signals are generated using amplifiers 20 which operate at extremely high power levels, The operation of the amplifiers 20 at these high power levels and the operation of other components within the system 10 at these high power levels increases the rate at which the amplifiers 20 and other components fail. Thus, in addition to the high losses and high cost, the system 10 also suffers from a relatively high failure rate.
An additional disadvantage associated with the system 10 is that the losses associated with the feeder lines 22 limit the maximum height of the tower 14. The amplifiers 20 can realistically only operate at certain power levels and operation above these levels introduce an unacceptable rate of failure or an unacceptable cost. Because of this practical limitation on signal power level, the feeder lines 22 must be restricted to a certain length if the antennas 16 are to receive signals at a sufficiently high power level. This limitation in feeder line length, in turn, translates into a height restriction for the tower 14. Since the height of the tower 14 effects the propagation area for the antennas 16 and the size of the cell, the limitation in tower height 14 also results in a need for a greater number of cells within the cellular network, thereby increasing the costs of the cellular network.
A further disadvantage of the system 10 is that it is often difficult finding suitable real estate for the base station 12. In urban areas, for instance, the tower 14 may need to be placed on or in a building or on some other existing structure. To minimize the distance between the antennas 16 and the amplifiers 20, the base station 10 is preferably located in close proximity to the tower 14. A close location to the tower 14, however, is often not possible and the closest suitable location within a building which is not presently being utilized for the amplifiers 20 may be in an underground closet or similar distant location. Once a location for the base station 12 has been found, the location must then be modified to provide the necessary wiring, cooling and ventilation, and security measures for the electronics within the base station 12. The need for a large area to house and cool the base station 12 renders it difficult and expensive to place the base station 12 in a building. Some rural areas also present similar difficulties in positioning the base station 12 close to the tower 14 in view of the harsh terrain that may surround the tower 14.